Partition exchange

I am in the process of moving the partitions of a table from ASSM to a
non-assm tablespace using the partition exchange method (create non
partitioned table, indexes, constraints etc.). When I do the alter table
exchange partition including indexes without validation command it runs for
hours, doing a full table scan on all the partitions of the partitioned
table which takes many hours. I understood that just the data dictionary is
updated, and it should take a few seconds. Primary Wait event is db file
scattered read. Can anyone shed some light on this phenomenon. DB version is
10.2.0.3 and is running on Solaris 10.